This section provides information related to the present disclosure which is not necessarily prior art.
In general, light scattering phenomena involve interactions between light and materials. One example of this natural phenomenon is light scattering within homogeneous media, such as in clouds, milk, human cellular tissues, and particles. Light propagation properties are affected not only by the refractive index difference between a light transmissible medium (which can be used as an industrial material to implement light diffusion) and light scattering particles, but also by the cross-sectional morphology of the light scattering particles, such as the size and volume fraction of the light scattering particles. Consequently, light transmittance and light diffusion negatively affect each other. Based on this scientific fact, many attempts have been made to overcome the above-mentioned disadvantage; however, the inventors are not aware of any technique to produce a polymeric material which transmits and scatters light simultaneously and concertedly.
The widely known theories of light scattering which use cross-sectional morphology were posed by Rayleigh (1899) and Mie (1908). Rayleigh's theory applies when the size of light scattering particles is smaller than the wavelength of light. Mie's theory applies when the size of light scattering particles is larger than the wavelength of light. For light diffusion to occur without wavelength conversion, Mie's theory predicts that the size of light scattering particles must be larger than the wavelength of light. Accordingly, prior publications have commonly focused on optimizing the size and volume fraction of light scattering particles.
The first product which used light diffusion by controlling the internal structure of a light diffusing medium is believed to be a glass lighting apparatus. In 1933, Henry presented a theory based on experimental data which showed that it was possible to acquire 30 times more light diffusion intensity when the internal crystalline structure of the glass was adjusted instead of the surface roughness of glass. As a result, most panel lighting apparatuses today are manufactured in such a way that the internal crystalline structure of glass is adjusted during the cooling process.
Recently, selective light sources have been used to improve the efficiency of light transmittance and light diffusion. This is possible because of current light emitting display devices, such as liquid crystal displays (LCDs) and light emitting diodes (LEDs), which are themselves direct applications of polymeric films. For example, a plurality of polymeric films, such as light guide plates (LGPs), diffusion sheets, and prism sheets, can be configured together to form an LCD backlight unit.
Nowadays, there are many efforts to simplify such complex configurations. These simplifications are expected to become used widely. Such efforts include a method in which light diffusion efficiency is improved through higher light scattering with a lower particle volume fraction, a result achieved by increasing the refractive index between a polymeric light transmissible medium and light scattering particles. A commonly recognized problem is the deterioration of brightness, brightness uniformity, and hue uniformity caused by irregular dispersion of light scattering particles used to control cross-sectional morphology.
In another approach, Barthelemy et al. presented a theory in Nature (A Lévy Flight for Light, May 2008, Nature) describing that light sometimes diffused according to a ‘Lévy flight’. Based on this theory, they propose that it is necessary to understand optical materials based on light diffusion behavior according to cross-sectional morphology in order to acquire uniform light diffusion efficiency, because light transmittance and light scattering depend upon the optical material's cross-sectional morphology and the refractive index difference of its constituent materials.
In another example, a large refractive index difference was used to successfully control the cross-sectional morphology of optical fibers; however, this is unrelated to the above-mentioned light emitting display devices. Furthermore, optical recording devices, light concentrator devices, or any optical device which intentionally triggers an optical nonlinearity or makes use of a nonlinearity, are unrelated to the examples described in the present disclosure.
Prior publications commonly rely only on the use of a large refractive index difference to devise light diffusion materials, though methods may vary. Through such methods, there have been a number of attempts to ensure simultaneous light transmittance and light scattering. However, no material which shows 90% or higher in both total light transmittance (transmittance of light from a straight path) and haze (an efficiency index of light diffusion by light scattering) has been reported yet.
Recently, a method has been reported in which a protrusion or a dent is made in relief or in intaglio on polymeric films or polymeric sheets to induce the concentration or diffusion of light. This method is used instead of adding light scattering particles into light transmissible media, signaling a departure from the methods mentioned above. The biggest problem with such methods is that it is impractical to maintain consistent light quality. For manufacture, a so-called imprint method is utilized, wherein a mold roll (the equipment to manufacture such films or sheets from polymer material) is embossed so that when the polymeric material is fed through the equipment, the polymeric material is molded into a polymeric film by ultraviolet (UV) radiation. Quality is limited by difficult separation of the polymeric film from the mold equipment after curing. If any additives are used to facilitate the separation, the desired level of quality may not be achieved. Equipment maintenance costs can soar when large-sized light emitting display devices are manufactured. More importantly, if such a new method were practiced, it may only satisfy the lowest of industrial requirements, for it is difficult to obtain compatibility between light transmittance and light diffusion, as addressed previously.
In addition, there has been an attempt to fundamentally overcome the above-mentioned drawbacks by inserting large amount of air bubbles into polymeric films. In this case, there is a big improvement in the maximum value of light transmittance from an existing level of about 45% to about 75% and the haze value has also increased to about 80% or higher. Therefore, it is expected that such polymeric films can be used in parts such as recent LED backlight units; however, this method is still limited by the previously reported partial wavelength conversion of white light caused by inconsistent air bubble generation. More importantly, heat resistance of the polymers problematically decreases after long-term light exposure. Actual quality control is limited because the boundaries between neighboring air bubbles may move because of high air bubble content within polymeric films.
As mentioned above, prior publications report a low level of compatibility, especially between simultaneous high light transmittance and light diffusion, which has various problems and limitations in practical stage in using polymers as optical materials. Therefore, the inventors have recognized a great need for improvement in polymeric films.